There are many methods for applying heat treatments to articles of manufacture. One method for heat treating an article of manufacture is to place it in a hot-air furnace. An article may be placed within a hot-air furnace and kept at a temperature for a specified period of time to provide certain heat treatments. There are problems with hot-air furnaces. Among the problems is that the heat-treating processes take a long time and therefore, the use of hot-air furnaces is not conducive to a manufacturing line having a high throughput. Another problem is that it is difficult to hold a temperature very closely within a hot-air furnace. The temperature can fluctuate as much as 50.degree. C. With such fluctuations, it is difficult to get a uniform heat-treated article from a hot-air furnace. In other words, the properties of the heat-treated article also vary because the temperature within the hot-air furnace varies.
Articles are quenched to preserve or maintain a desirable grain structure. The typical method for quenching is to take the article and place it into a water bath. The water bath is typically at a temperature of at least 180.degree. F. Placing an article having a much higher temperature into the water or liquid bath results in some of the water or liquid turning into a vapor. During the quenching process, portions of the article will be cooled by liquid and other portions will be cooled by the vapor. The liquid and vapor carry heat away from the article at different rates. In short, articles which are quenched in liquid or water baths have uneven cooling. Uneven cooling during a quench may result in distortions in the article. This is one of the problems associated with the traditional approaches to quenching a part or article.
Other methods for applying heat or removing heat from articles use a fluidized-bed furnace. The fluidized-bed furnace can be used to heat treat, quench or age a part. The main elements of a fluidized-bed furnace include a retort, a heating system and an air introduction system, such as an air manifold. In the bottom of the furnace is an air introduction system, through which a flow of fluidizing gas, generally air, enters the retort and fluidizes the fluidizing medium. The retort is filled with a fluidizing medium, such as fine-grained sand or stainless steel. A flow of fluidizing air enters the retort at the bottom and sets the particles in the medium in suspended motion. The fluidized-bed furnace uses the properties of a fluidized medium to transfer heat to or away from an article under treatment. The fluidized medium is physically similar to a liquid. The fluidizing medium is inert. As an inert medium that acts similar to a liquid, heat transfer to an article is usually rapid. Thus, the heat-transfer properties are superior to the heat-transfer properties of a hot air furnace. The temperature of the fluidizing medium and the time the part is exposed to the medium determines the type of treatment. A single retort furnace can be set up to deliver various heat treatments by changing the temperature setting and the atmosphere mixture. Although a single fluidizing medium can be used to heat treat, quench or age an article it should be pointed out that changing temperatures of the fluidization bed so that a different treatment can be applied typically takes from 10 to 24 hours.
There are problems with todays fluidizing beds. Some of the problems relate to heating the fluidized bed. The heating of the fluidized medium may not be uniform. In many fluidized beds, the heaters are placed on the outside of the retort. When placed outside the retort, the heating of the fluidized medium is not as uniform as when the heaters are placed directly into the retort. U.S. Pat. Nos. 4,752,061 and 5,294,095 both teach placing a heating element directly into the retort. In each of these particular fluidized beds, infrared lamps are the heating elements which are placed within the fluidized bed. There are problems with placing infrared lamps into a fluidized bed. First of all, the infrared lamps are fragile and will break easily. Secondly, ceramic seals used between the outside of the retort and the infrared lamp may break and allow sand to escape from the retort. Another problem is vitrification where the sand makes a portion of the quartz sleeve opaque. The lamp then overheats and the infrared lamp fails. When the heating element fails in the fluidized bed, the entire fluidized bed must be cooled down, the fluidizing medium must be removed, and the heating element must be replaced. The process of replacing a heating element takes a considerable amount of time. This time is wasted production time.
Another problem with current fluidized beds is that they use batch processing. A batch of parts are loaded into a basket and then lowered into the fluidized bed. The baskets are then loaded into the fluidized-bed furnace from above to load a number of individual parts or components into the fluidized bed. The baskets, racks or individual components are held within the fluidized bed for a prescribed or desired amount of time and then removed from the fluidized bed. Another batch of articles or parts are then heat treated using the same method. One problem associated with batch processing is that the parts or articles within the baskets are spaced so closely together that the fluidizing medium may not get to all the surfaces of the part or article. The result is uneven heating or cooling of the article.
Still another problem with current fluidized beds is that they waste energy. The fluidizing medium is heated to a prescribed temperature necessary to accomplish a heat-treatment process. Gas or air is heated as it passes through the fluidizing medium. After air passes through the fluidizing bed it is exhausted and a new supply of air is heated. Constantly heating the air robs energy from the fluidized bed. This is quite costly.
In order to save costs on energy, some fluidizing beds collapse the bed which means that air is no longer passed through the bed while the heat treatment is taking place. In other words, after a batch of parts or articles are placed in the bed, the fluidizing air is shut off and the bed collapses around the parts or articles while the heat treatment takes place. This is one way to save energy. Collapsing the bed requires batch processing.
Casting a part and removing it from a mold typically takes 5 to 12 minutes. Parts or articles can be output from a casting machine at a rate of one part or casting per five minutes. Current methods for treating parts require long treatment times. For example, heat treating a part may take as long as 12 hours in an open-air furnace. Aging a part may take as long as 8 hours. Because of the long cycle times associated with treating parts and the fact that castings are produced at a higher rate, batch processing seems necessary. One of the byproducts of batch processing is that it precludes the formation of a manufacturing line that can use continuous processing. Another byproduct of batch processing is that it requires the castings to be stacked and cooled before a batch can be assembled for treatment. This introduces inefficiencies in terms of using the heat associated with the manufacturing process. Simply put, allowing the casting to cool wastes thermal energy. The part must be reheated before undergoing further treatment, such as heat treating, quenching and aging a part.
In addition to an inefficient use of thermal energy, in some alloys cooling of the parts may also place additional stresses or reduce the quality of the finished part. For example, in a part including an alloy of aluminum and magnesium include mag suicides. When the part comes from being cast, the mag silicides are in suspension. Cooling the part below 750.degree. F. results in the separation of the mag silicides from the structure of the part. The mag silicides do not remain in suspension below 750.degree. F. Allowing the part to cool to a temperature where the mag suicides are no longer in suspension may result in a poorer quality finished part. Upon reheating, the mag silicides are put into suspension once again, but it is felt that the properties of a part that is cooled and then reheated may be lesser than a part that never cooled to a point where the mag silicides went out of suspension.
Therefore, it would be advantageous if a casting could be removed from a mold and then placed directly into a heat treatment process at an elevated temperature. The manufacturing process would be more efficient. In addition, some of the properties of the part may be enhanced since the part from the mold did not cool to room temperature. It would also be advantageous if the cycle times for heat treating, quenching and cooling a part could be reduced since this would eliminate a major bottleneck in the production flow. Current cycle times for heat treating, quenching, and aging can be as long as 24 hours and must be undertaken off-line in labor-intensive batch processing steps. The current technology also uses high amounts of energy for processing at temperatures of 1000.degree. F. or more, with consequent heavy air emissions of various pollutants. In addition, when a part is sand cast, contaminated sand cores must be removed in a separate process and disposed of as waste when using current heat treatment, quenching and aging processes. This not only adds time, but also adds to the waste produced by the processing.
The root cause of high energy use and air emissions in heat treatment is the inefficiency of batch processing systems in which components are stacked in baskets and then placed in a convection or vacuum furnace. The long cycle time is required to assure that the entire component, no matter how complex or varied in thicknesses, is heated to sufficient temperature to alter the metal's molecular structure. To assure that inner surfaces and cores of thicker elements reach the specified temperature, the components must be left in the furnace for as long as 8-12 hours, with careful control of temperature to assure that eutectic melting does not occur on outer surfaces. Even recent advances, enabling gas-fired rather than electrical furnaces and other fluidized bed systems, still reduce heat treatment cycle time to no less than 4 hours.
There is also a need for a process which enables heat treatment, quenching and aging of a article, such as an aluminum casting, in a shortened cycle time, thereby eliminating the current bottleneck in manufacturing cast parts. Such a process would enable just-in- time manufacturing methods and allow for continuous in-line processing.
Thus, there is a need for a process which allows for heat-treating, quenching and aging on a manufacturing line having a continuous flow of parts. There is also a need for a process which will be energy efficient and ecologically friendly. There is also a need for a process which will provide for clean parts after the part or article has passed through the fluidizing bed. There is also a need for a reliable process which can hold temperatures very accurately and which will not break down frequently, thereby causing a costly shutdown of a manufacturing line. In addition, there is a need for a process which decors sand cast parts without producing a sand core waste product.